Bigger prop.. over revving is worse??

[82rude]

Correct jimmbo.He can still go on here and they show the 70 merc he has .Even show factory merc props.Maybe im missing something in his description but the motor shouldnt be reving as it feels like it thats why i suggested spun hub or even maybe a damaged or stripped shaft as he did say the old one was cut off.

 

[Fed]

Thanks for the heads up jimmbo by chance I actually owned a couple of 65HP (3 cyl blue bands) which I upgraded to 80HP (4 cyl red bands) with the larger gear cases so I should have been more on the ball.

 

[camarokidz28]

I ran it through the Mercury Prop Selector (had to use a 75HP due to year model).
It came back with 5 different props... each one was a 19" pitch!

Allowing for a 5HP reduction down to 70HP you should be looking for around a 17" prop pitch.

Don't get confused, props are measured by pitch & diameter.
The diameter is hopefully obvious to you.
The pitch is a measurement in INCHES which is the theoretical (no slip) distance a prop would travel forward with 1 revolution.

​From what I was seeing the 70 hp was basically replaced by the 60 hp when they changed from gross to net power.

​I did a lot of searching last night. and I think that what hurt me was actually the diameter drop. not the pitch. My motor sits pretty high on the transom. So the combination of smaller prop and more aggressive is just causing ventilation to happen. I'm going to try going to the largest diameter 13p or 14p that I can find.

 

[Fed]

I'd test it for a spun hub before spending any more money.

 

[82rude]

Iboats shows the biggest dia prop for you is 11 1/2 .Your gearcase will only allow for a set max dia you will never fit a later model 13 or 14 inch dia as the clearences are much larger on those motors gearcases.So im trying to get this straight.When you say your motor sits high and it revs high but goes only 14mph are you saying its badly ventilating?If thats the case lower your motor a bit.If on the other hand its over reving but not blowing out then its a spun prop.

 

[camarokidz28]

Iboats shows the biggest dia prop for you is 11 1/2 .Your gearcase will only allow for a set max dia you will never fit a later model 13 or 14 inch dia as the clearences are much larger on those motors gearcases.So im trying to get this straight.When you say your motor sits high and it revs high but goes only 14mph are you saying its badly ventilating?If thats the case lower your motor a bit.If on the other hand its over reving but not blowing out then its a spun prop.

​Unfortunately it's a low as I can go with it. The only way to lower it would be to get a longer shaft motor. If I put a straight edge on the bottom of the boat and run it back to the prop the tip of the prop is actually above the bottom of the boat. So I think that losing an in on the prop diameter is what killed me. I can test it for a spun hub. But I really didn't see any signs at all that it had spun. I won't be able to get it to the river until probably Tuesday at the earliest

 

[ondarvr]

Some prop designs can handle being mounted higher better than others.

 

[82rude]

Telling a spun hub by eye is very tricky.Doing the test as described is infallible.Mark and duplicate your issues and check.Every prop ive ever tested that was reving up but going nowhere's was spun almost without exception so far.The only exception was a stick that got jammed between the prop and casing and believe it or not it mimicked a spun hub but when the stick was removed everything was perfect.

 

[Sea Rider]

If prop catches a small piece of floating plastic without noticing it will behave as a spun prop. Props don't spun hubs as flicking your fingers, usually the vulcanized hub will take its time for prop body to spin 100% freely and become kaputt,. OB's not properly height seated will ventilate due to poor water bed height being inappropriate, prop will not grip well on insufficient amount of water. Try swiming like a dog with both hands while fingers are widely open...

Happy Boating

 

[jimmbo]

​Unfortunately it's a low as I can go with it. The only way to lower it would be to get a longer shaft motor. If I put a straight edge on the bottom of the boat and run it back to the prop the tip of the prop is actually above the bottom of the boat. So I think that losing an in on the prop diameter is what killed me. I can test it for a spun hub. But I really didn't see any signs at all that it had spun. I won't be able to get it to the river until probably Tuesday at the earliest

How about posting a picture(s), taken from the side, level with the bottom of the boat so we can get an idea of how high your engine is mounted

 

[Fed]

I once had a good hub, I flicked my fingers and the hub was spun.
Now I never flick my fingers until I'm on the plane.

 

[Sea Rider]

I once had a good hub, I flicked my fingers and the hub was spun.
Now I never flick my fingers until I'm on the plane.

Better have a second spare prop on board, you'll never know when props are going straight towards kaputt land...

Happy Boating

 

[camarokidz28]

UPDATE**
Never tested for spun hub. But I bought a 11 3/5" x 13p and the boat now goes again... Boat struggles to plane still and and will only reach about 4800 rpm right now

 

[jimmbo]

UPDATE**
Never tested for spun hub. But I bought a 11 3/5" x 13p and the boat now goes again... Boat struggles to plane still and and will only reach about 4800 rpm right now

It will struggle as the prop has too much pitch as witnessed by the 4800 rpm. Which is below the 5000 - 5500 that engine is supposed to run at

 

[camarokidz28]

It will struggle as the prop has too much pitch as witnessed by the 4800 rpm. Which is below the 5000 - 5500 that engine is supposed to run at

​I'm gonna do a tune up on it, new plugs, clean the carbs, and fuel pump rebuild. Then I'll check timing and sync everything. Hopefully I can get the rpm up.. But I think ultimately I'll probably go back down to a 12p prop. Really glad I didn't go 14p or 15p even, that would have put me dead in the water. I got 31 MPH GPS out of this set up at 4800 RPM moving upstream.

 

[Sea Rider]

Anyway, if with a 12 prop achieved just 4800 wot rpm, engine is running at the lugging side of the wot rpm range, which is bad. If engine didn't get any better after checking all what's mentioned, need to go to at least 1-2 less pitches to pull wot revs higher to around middle or max wot range. Or modify current 12 pitch to a 11-10 pitch ?

Happy Boating